Project Summary Fragile X Syndrome (FXS) is the most common type of mental retardation that can be linked to a single gene mutation. FXS patients exhibit many behavioral alterations, as well as abnormal development of synapses in the brain. Astrocytes, the major type of glia in the mammalian brain, regulate synaptic and neuronal functions and are implicated in many developmental and degenerative neurological diseases. The goal of this proposal is to determine the roles of astrocytes in FXS. Combining mouse genetics, live imaging, synaptic molecular profiling, behavioral analyses and pharmacological intervention, we propose 3 aims. In Aim 1, we study how astrocytic deletion of Fragile X Mental Retardation Protein (FMRP) contributes to the synaptic and behavioral defects observed in mice. We will generate transgenic mice in which FMRP is selectively deleted or exclusively expressed in astrocytes. We will then compare the synaptic and behavioral phenotypes of these mice with those of wild-type controls and FMRP full knockout mice. Aim 2 builds upon our earlier observation that astrocyte-specific FMRP knockout mice have increased production of immature dendritic spines of cortical neurons. We will combine in vivo imaging with mathematical modeling and synaptic proteomic imaging to address how astrocytic deletion of FMRP affects the spatial distribution of spinogenesis on excitatory cortical neurons, as well as synaptic/peri-synaptic neuronal and astrocytic protein expression of newly formed spines. In Aim 3, we examine the functional changes of synapses in mice in which FMRP is selectively deleted in astrocytes. In particular, we will examine how glial glutamate uptake and synaptic glutamate concentration are affected in astrocyte-specific FMRP knockout mice, and determine if correcting abnormal glutamate uptake alleviates the dendritic spine defects in these mice. The proposed work will be the first systematic in vivo study investigating astrocytic contribution to FXS. By examining the role of astrocytes in the neuropathology of FXS, these studies will advance our understanding of the disease and potentially point out new therapeutic targets.